A comparison between two calcite-rich deposits in palaeoproterozoic dolomitic units of the Transvaal supergroup: beestekraal; North West province and lime acres; Northern Cape

• Main Author: Ntibane, Thabile Malecia
• Format: Others
• Language: en
• Published: 2019
• Online Access: https://hdl.handle.net/10539/26733

A thesis/dissertation submitted in partial fulfilment of the requirements for the degree Master of Science in Economic Geology, University of the Witwatersrand Faculty of Science School of Geosciences, 2018 === The Malmani and Campbellrand Subgroups have small zones of calcite that did not undergo dolomitisation when it took place within the Transvaal Supergroup. To get the calcite rich zonations of limestone the Malmani and Campbellrand Subgroups may have avoided dolomitisation and dedolomitisation during the geological events of the Transvaal Supergroup. This project integrated geological mapping, petrology, microprobe analysis, mineral identification, bulk geochemistry and point geochemistry in an attempt to understand and compare the calcite rich limestone zonations that occur in the Malmani and Campbellrand Subgroups. The calcite rich limestone either preexisted the deformation of the Transvaal Supergroup or resulted from dedolomitisation of the deformed Transvaal Supergroup. The Campbellrand Subgroup of the Griqualand West Basin is composed of shallow shelf carbonates that were deposited under shallow subtidal conditions, whereas the Malmani Subgroup has the Oaktree Formation which is generally dolomitic, except in the Crocodile River fragment. The sedimentary structures on the Lime Acres Member are interbedded by fine grained limestone, chert and dolomitic matrix. The Lime Acres Member has two NS fault which have high grade limestone between them and on the far western side of the faults, dolomite ore deposit lies on the eastern side. The limestone structures and textures which are found on the western side of the fault 2 (F2) and eastern side of the fault 1 (F1) in Lime Acres were well preserved. The dip is at about 3⁰ -7⁰ to the west. The Oaktree Formation in the Malmani Subgroup hosts the carbonate deposit at Beestekraal which comprises dolomitic limestone with chert-rich and chert-poor zones within the limestone. The deposit dips to the west at an angle of about 33⁰ and strikes NS. There are four faults that cut across the deposit and these are striking WE. The faults form normal faults that are fairly steep. Folding is noted in the hanging wall where the WE faults that cut across the deposit. The Campbellrand Subgroup deposit is interbedded with limestone and dolomites. The dolomite is coarse grained and can be classified as replacement rock that formed by diagenetic replacement of the older limestones, while the limestone is fine-crystalline and can be classified as primary limestone i.e. not dedolomitised. The dolomite crystals show twinning and cross-cuttings of cleavage planes under a microscope and has proven to have Mg/Ca value of 0.8 -0.9 with a shallow dip. The characteristics that affect each site are different as Lime Acres has preserved sedimentary structures such as stylolite and stromatolites, whereas such structures are lacking at Beestekraal, where there is a dominance of calcite rich veinlets. The calcite veinlets have upgraded the dolomites at Beestekraal, while the chert downgrades the deposit. Faults, calcite veinlets and quartz inclusions were post depositional of the limestone and dolomite deposit. No evidence was found that the faults could have eroded, downgrade or upgraded the limestone zones. The faults fissures are filled with reddish-clay at Lime Acres and deformed chert at Beestekraal. Re-entrance angles, twinning, fenestrae and cross-cutting cleavage planes were noted in both electron microprobe analysis and petrology analysis. These features are consistent with dolomite structures. There is evidence of three geological events that have occurred on both site where; • Deposition of the magnesium-calcite occurred; iv • Some silica and calcite veinlets were introduced by fluids, the fluid introduction put pressure on the deposit which resulted in compaction. Re-entrance angles formed during compaction and left the prominent calcite veinlets. The calcite veins only appear in Beestekraal; • Faulting occurred post the formation earlier compaction evensts. The faulting caused compression of the existing structures and resulted in wavy stylolites at Lime Acres, deformation along the faults and mud fissures being introduced to the joints. There was uniformity between the X-Ray Fluorescence, X-Ray Diffraction, and electron microprobe analysis results. Both sites have similar mineral composition which are rich in calcium with little deformation. The study has found that the mining areas have somehow not been altered during the dolomitasation phase that affected the Transvaal Supergroup rocks. The pockets of high calcium deposits can be classified as primary limestone, which was somehow preserved during the dolomitisation phase. Post depositional alterations had two different effects on the sites. At Lime Acres the chert, which filled the contacts between the different bedding, was compressed due to high pressure from the overlying material and faulting. The traces of chert have been noted as stylolite and small traceable carbon partings/bands within the limestone and dolomites. At Beestekraal there is infill of iron rich and chert material, which occurs within the cleavage planes and contacts. Both sites have significantly increased calcium content with low magnesium content in production zones, having little impurities from the chert and dolomites that overlie the limestone. The Mg/Ca ratios showed that for all zones used for cement and lime production the Mg/Ca ratios are below 0.6 and 0.1 respectively. Mg/Ca ratios closer to 1 and above are not economically viable for lime and cement production. The low Mg/Ca ratios indicate that the dolomites are more of magnesium calcite content and of primary depositional environment than secondary. === XL2019